这是我的表定义：

create table User (
    userUuid text not null primary key,
    username text not null,
    thisUserBlockedCurrentUser int not null,
    currentUserBlockedThisUserTsCreated int not null,
    searchScreenScore int,
    recentSearchedTsCreated int,
    friends int not null
);

create index User_X on User(thisUserBlockedCurrentUser, friends);

这是我的查询+计划：

explain query plan
select *
from (select User.*
      from User
      where friends = 1
        and User.currentUserBlockedThisUserTsCreated is null
        and User.thisUserBlockedCurrentUser = 0
        and User.username != ''
      union
      select User.*
      from User
      where recentSearchedTsCreated is not null
        and User.currentUserBlockedThisUserTsCreated is null
        and User.thisUserBlockedCurrentUser = 0
        and User.username != '')
order by case when friends = 1 then -2 when recentSearchedTsCreated is not null then -1 else searchScreenScore end,
         username;

CO-ROUTINE (subquery-2)
COMPOUND QUERY
LEFT-MOST SUBQUERY
SEARCH User USING INDEX User_X (thisUserBlockedCurrentUser=? AND friends=?)
UNION USING TEMP B-TREE
SEARCH User USING INDEX User_X (thisUserBlockedCurrentUser=?)
SCAN (subquery-2)
USE TEMP B-TREE FOR ORDER BY

所以使用了索引，但是在order by中仍然有一个扫描和一个b树。我试着通过添加更多的索引来摆脱它们，但是我没有让它工作。
有没有关于去掉scan的索引的想法？